Cobalt(ii)-catalyzed benzylic oxidations with potassium persulfate in TFA/TFAA

Article abstract of DOI:10.1039/c9ra03346g

A cobalt-catalyzed C(sp³)-H oxygenation reaction to furnish aldehyde was herein reported. This transformation demonstrated high chemo-selectivity, and tolerated various methylarenes bearing electron-withdrawing substituents. This reaction provided rapid access to diverse aldehydes form methylarenes. Notably, TFA/TFAA was used for the first time as a mixed solvent in cobalt-catalyzed oxygenation of benzylic methylenes.

Full text of DOI:10.1039/c9ra03346g

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Cobalt(II)-catalyzed benzylic oxidations with
potassium persulfate in TFA/TFAA†
Cite this: RSC Adv., 2019, 9, 20879
a
b
a
Tianlei Li, ‡a Jishun Li,‡abc Zihao Zhu, Weidong Pan and Song Wu
*
*
A cobalt-catalyzed C(sp3)–H oxygenation reaction to furnish aldehyde was herein reported. This transformation
demonstrated high chemo-selectivity, and tolerated various methylarenes bearing electron-withdrawing
substituents. This reaction provided rapid access to diverse aldehydes form methylarenes. Notably, TFA/TFAA
was used for the first time as a mixed solvent in cobalt-catalyzed oxygenation of benzylic methylenes.
Received 5th May 2019
Accepted 27th June 2019
DOI: 10.1039/c9ra03346g
rsc.li/rsc-advances
cobalt(II)/N-hydroxyphthalimide (NHPI) catalyst system.17
Recently, Pappo and co-workers also revealed the unique
Introduction
chemoselectivity for aerobic autoxidation of methylarenes to
benzaldehydes based on N-hydroxyphthalimide (NHPI) and
cobalt(II) acetate in 1,1,1,3,3,3-hexauoropropan-2-ol (HFIP).
The uorinated alcohol and benzaldehyde may form a H-bond
adduct that markedly slow down H-abstraction of the alde-
hydic C–H bond.18 It is worth mentioning that this strategy
address the long-standing selectivity problem of generating
benzaldehydes directly from methylarenes under sustainable
conditions.
Other cobalt-catalyzed systems such as TEMPO/Co(OAc)2,19
tert-butyl hydroperoxide/Co(acac)2,20 and oxone/Co(ClO4)2 (ref.
21) were developed for the allylic and benzylic oxidation of
alkylarenes and methylarenes. The desired ketones derivatives
Benzylic oxidative reaction is one of the most useful and
important C–H functionalization in synthetic organic chemistry
due to its wide application in the manufacture of pharmaceu-
ticals and chemicals,1 and these transformations are able to
construct various functional groups such as ketone, aldehyde,
hydroxyl, carboxylic, ester etc. Numerous approaches for the
C(sp3)–H benzylic oxygenation to the corresponding carbonyl
derivatives have been developed.1,2 Traditional methods
involving the use of stoichiometric amounts of metallic
reagents such as KMnO4, Cr(VI) or potassium dichromate2a are
common. In the past few decades, many efforts have been
devoted to develop oxidation processes catalyzed by transition
metals, such as Mn,3 Cr,4 Fe,5 Cu,6 Bi,7 Rh,8 Ru,9 Pd,10 Re,11 Au12
and V13 species among others.14, and fruitful progresses have
been achieved.
With the development of C–H functionalization that cata-
lyzed by low-cost and environmentally benign rst-row tran-
sition metals,15 various cobalt-catalyzed direct benzylic or
allylic oxidation reactions have been reported (Scheme 1).16–21
Pioneered by Ishii et. al., cobalt-catalyzed C–H(sp3) oxidation
with N-hydroxyphthalimide (NHPI) has proven to be a valuable
method for the preparation of ketones from alkylarenes and
benzoic acids form methylarenes.16 Following this strategy,
Stahl's group reported that benzylic methylene groups in
pharmaceutically relevant heterocyclic substrates could be
effectively converted into the corresponding ketones by the
aState Key Laboratory of Bioactive Substance and Function of Natural Medicines,
Institute of Materia Medica, Peking Union Medical College, Chinese Academy of
Medical Sciences, Beijing, 100005, China. E-mail: ws@imm.ac.cn
bState Key Laboratory of Functions and Applications of Medicinal Plants, Guizhou
Medcial University, Guiyang 550014, China. E-mail: wdpan@163.com
cGuizhou University of Traditional Chinese Medicine, Huaxi University Town,
Guiyang, Guizhou, 550025, China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c9ra03346g
Scheme
1 Cobalt-catalyzed C–H functionalization (benzylic and
‡ These authors contributed equally to this work.
allylic oxidation reactions).
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are effectively obtained under these conditions. However, the
large excess of oxidants were oen required. In addition, the
typically more reactive aldehydes are generally difficult to
prepare directly from the corresponding methylarenes under
most of the above methods.
Results and discussion
We commenced our study with the benzylic oxidation reaction
of 4-methylbenzophenone (1a) in the presence of 20 mol% of
Co(OAc)2$4H2O as the catalyst, 2.0 equiv. of K2S2O8 as the
ꢀ
Despite these signicant advancements made in the area of
low-valent cobalt-catalyzed benzylic oxygenation, but the
selective oxidation of methylarenes to form benzaldehydes is
still very challenging. Moreover, the efficiency of C(sp3)–H
benzylic oxygenation were severely restricted by the electronic
property of additional substituents on the aryl ring. With the
aryl rings bearing electron-donating groups, the correspond-
ing C(sp3)–H benzylic oxygenation smoothly occurred. While
electron-withdrawing groups on the aryl rings generally
hampered this transformation.16–21 Up to date, the direct
C(sp3)–H benzylic oxygenation reactions for the preparation of
benzaldehyde that could tolerate with electron-withdrawing
groups on the aryl ring are still rare. In consequence, the
discovery of a new method for direct benzylic oxidation toler-
ated with electron-withdrawing groups on the aryl ring would
be of considerable importance. Herein, we report a new
method for the direct benzylic C(sp3)–H bond oxidation
through cobalt catalysis to afford a series of corresponding
aldehydes. This method manifests highly chemo-selectivity,
and tolerates with various methylarenes bearing electron-
withdrawing substituents. The reaction employs the low cost
Co(OAc)2$4H2O as the catalyst, K2S2O8 as the oxidant and TFA/
TFAA as the co-solvent.
oxidant in TFA/TFAA (6 : 4) co-solvent system at 100 C. Under
this condition, 2a could be obtained in 12% isolated yields.
Initially, we examined this reaction in the presence of a mixted
solvents of TFA/TFAA, the aldehyde product 2a was observed in
less than 5% yield with the ratio 5 : 5 of TFA/TFAA (entry 2) and
36% yield with the ratio 9 : 1 of TFA/TFAA at 40 ꢀC (entry 5). But
the reaction did not occur in TFA as single solvent (entry 6). The
results suggest that the ratio of TFA/TFAA play a key role in this
reaction. The high proportion of TFA could dramatically
improve the conversion of benzylic oxidation, and TFAA might
be involved in the initiation step of cobalt-catalyzed C–H bond
cleavage process.8c,22 When the reaction was performed at 80 ꢀC,
the yield dramatically increased to 78% (entry 9). Further
experiments revealed that other cobalt catalysts such as CoBr2
and CoF3 were inferior to Co(OAc)2$4H2O (entry 10–12), and the
benzylic bromination product was observed when CoBr2 was
used as a catalyst. As a control reaction, the benzylic oxidation
could not occur in the absence of Co(OAc)2$4H2O (entry 13).
Among the oxidants investigated, Na2S2O8 was less efficient
than K2S2O8 (entry 10). (NH4)2S2O8 was much more powerful
than K2S2O8. However, the decomposition was observed when
(NH4)2S2O8 was used instead of K2S2O8 (entry 11). The
commonly applied oxidants NFSI and PhI(OAc)2 were proved to
be inactive (entry 7–8). In general, 20 mol% of Co(OAc)2$4H2O
was enough to catalyze this transformation, the process
Table 1 Optimizing the reaction conditions of benzylic oxygenationa
TFA/TFAA
Entry
Cobalt source (mol%)
Oxidants
ratio
Temperature
100 ꢀ
Yield
1
2
3
4
5
6
7
8
Co(OAc)2$4H2O
Co(OAc)2$4H2O
Co(OAc)2$4H2O
Co(OAc)2$4H2O
Co(OAc)2$4H2O
Co(OAc)2$4H2O
Co(OAc)2$4H2O
Co(OAc)2$4H2O
Co(OAc)2$4H2O
Co(OAc)2$4H2O
Co(OAc)2$4H2O
CoCl2
K2K2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2o8
NFSI
Phl(OAc)2
K2S2o8
Na2S2O8
(NH4)2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
6 : 4
5 : 5
7 : 3
8 : 2
9 : 1
10 : 0
9 : 1
9 : 1
9 : 1
9 : 1
9 : 1
9 : 1
9 : 1
9 : 1
9 : 1
C
12%b
<5%c
21%c
29%c
36%b
<5%c
N.O.d
N.O.d
78%b
41%b
52%c
19%c
<5%c
15%c
N.O.d
40 ꢀ
40 ꢀ
40 ꢀ
40 ꢀ
50 ꢀ
80 ꢀ
80 ꢀ
80 ꢀ
80 ꢀ
80 ꢀ
80 ꢀ
80 ꢀ
80 ꢀ
80 ꢀ
C
C
C
C
C
C
C
C
C
C
C
C
C
C
9
10
11
12
13
14
15
CoF3
CoBr2
Without catalysis
Note: Reaction condition: 1 (0.1 mmol), cobalt catalyst (0.02 mmol), oxidant (0.15 mmol, 1.5 eq.), TFA and TFAA (1 mL). b Isolated yield. c When
a
the conversion are less than 50%, the conversion ratio was detected by H-NMR. d N.O. means no observation.
1
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typically went to completion within 10 h with 2.0 equivalents of
ꢀ
K2S2O8 at 80 C Table 1.
Having identied the optimal conditions for the direct
C(sp3)–H benzylic oxygenation, we set out to explore the
substrates scope for this new reaction. As illustrated in Scheme
2, a range of substituted benzylic substrates were investigated.
The scope of methylarenes was broad, and the transformation
was smoothly occurred to selectively generate the correspond-
ing aldehydes in moderate to good yields. Aryl groups with
different substituent groups, such as Cl, Br, NO2, acetyl, were
tolerated under the optimal conditions. The various substituted
methylbenzophenones (2a–2d) were successfully oxidized into
the desired aldehydes with good yields (78–86%). Interestingly,
we found that 4,40-dimethyl-benzophenone (2d) and methyl 3,5-
dimethylbenzoate (2k) were selectively oxidized to give mono-
oxidation products in 74% and 65% yields, respectively. To
Scheme
oxygenation.
3
Proposed mechanism cobalt(II)-catalyzed C(sp3)–H
withdrawing groups lead to the benzylic C(sp3)–H more inert.
It is worth pointing out that our method provides a new access
for the selective oxidation of the electron-decient methylar-
enes to prepare benzaldehydes. To explore the practical utility
of this C(sp3)–H benzylic oxygenation reaction, a gram-scale
reaction of methyl 5-bromo-2-methylbenzoate oxidation was
performed (Scheme 2) under the standard conditions. The
oxidation product 2i could be obtained in 69% yield with
20 mol% of cobalt catalyst, which demonstrated scalable and
practical of this protocol.
our delight,
a range of methylbenzoates and methyl-
propiophenones bearing with electron-withdrawing groups
(–NO2, –Br, –Cl) were smoothly transformed into the corre-
sponding aldehydes in moderate to good yields (61–70%).
Unfortunately, the electron-rich substituted methylarene
substrates, such as p-toluidine, 1-methoxy-4-methylbenzene
and p-cresol, could not be converted the aromatic aldehydes
(2u, 2v and 2w) due to some side-reactions (Friedel–Cras
reaction, acylation etc. see ESI†) under the optimal conditions.
It was revealed that the electronic property of substituents on
methylarene derivatives displayed important effects on the
reaction efficiency, and this strategy prefers to the electron-
decient methylarenes. As mentioned before, the electron-
decient methylarenes are difficult to be oxidized under the
previous reported conditions,2,18–21 because the electron-
Previous reports proposed that a SO4cꢁ radical was
generated in situ in the presence of cobalt.21,23 Based on
literatures,23 a plausible mechanism for this oxidation
process was proposed as shown in Scheme 3. Firstly, the
reactions involved the oxidation of Co2+ with peroxydisulfate
2ꢁ
(S2O8 ) to generate a SO4cc radical in situ.23a-b Then the
radical reacted with methylarenes to give the radical inter-
mediate A radical assisted by the TFA/TFAA solvents, fol-
lowed by the reduction of Co3+ to provide the intermediate B
that could be detected when the reaction was proceeded at
ambient temperature (see ESI†). Following with the second
C–H oxidation, the intermediate C was generated, which
could be further converted to the desired product aer
a hydrolytic process. Very recently, Fyokin and co-workers
demonstrated highly polar triuoroacetic acid could be an
efficient solvent for the metal-free aerobic NHPI-catalyzed
oxidations of toluene.24 In our study, the reaction cannot
occur when TFA/TFAA solvent system was replaced with
AcOH/Ac2O under the standard conditions.
Conclusions
In summary, we developed a new protocol for the preparation of
benzylaldehydes from the corresponding methylarenes by the
direct oxygenation of benzylic C(sp3)–H. The method shows
high chemo-selectivity, and tolerates with various electron-
withdrawing substituted arenes, esters and ketones. The TFA/
TFAA solvent is rst used as a mixed solvent in cobalt-
catalyzed oxidation of methylarenes to benzaldehydes.
Conflicts of interest
Scheme 2 Substrate scope of cobalt(II)-catalyzed C(sp3)–H benzylic
oxygenation.
There are no conicts to declare.
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Acknowledgements
This work was supported by National Natural Science Founda-
tion of China (Grant No: 81703364), Chinese Academy of
Medical Sciences-CAMS Innovation Fund for Medical Sciences,
Grant No: 2017-I2M-1-011, China Postdoctoral Science Foun-
dation (Grant No: 2016M590107). The authors thank Dr Tian-
long LAN and Dr Chao Zhang for helpful suggestions during the
preparation of this manuscript.
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